My current plan is to add this with the next restart.
The mechanics are as following:
The colors likely need GLSL shaders enabled in the client and might not show up on potato PCs. This doesn't require any client-side modifications. All unicorns will get a random color assigned (from the full RGB range, so 16777216 possible colors). Color is not inherited, each offspring will have it's own random color, regardless of parents. Unicorns are proud and independent animals and will not confine their color choices to the limits of genetics. If vanilla WU ever ads colors for unicorns this might need to be revisited.